- added optional constructor to sql.ColumnCollection

- added optional constructor to sql.ColumnCollection
- mapper sets its "primary_key" attribute to be the ultimately decided primary_key column collection post-compilation
- added compare() method to MapperProperty, defines a comparison operation of the columns represented by the property to some value
- all the above combines into today's controversial feature: saying query.select_by(somerelationname=someinstance) will create the join of the primary key columns represented by "somerelationname"'s mapper to the actual primary key in "someinstance".
- docs for the above